refactor(api): trailing newlines + hoist acks.py parser imports to top-level

Self-review nits from the router-split commit:
- All four new files lacked a trailing newline (PEP 8 / POSIX).
- acks.py was lazily importing ParseResult999 / serialize_999 inside
  get_ack_endpoint. Hoist to module-level — there's no import cycle
  (acks.py does not import from cyclone.api), so the imports are safe
  to do once.

No behavior change. Targeted tests (test_acks + test_health + test_api_gets)
still pass 41/41.

backend/src/cyclone/api_routers/acks.py

@@ -17,6 +17,8 @@ from fastapi import APIRouter, HTTPException, Query, Request
 from fastapi.responses import StreamingResponse
 
 from cyclone.api_helpers import ndjson_stream_list, wants_ndjson
+from cyclone.parsers.models_999 import ParseResult999
+from cyclone.parsers.serialize_999 import serialize_999
 from cyclone.store import store
 
 router = APIRouter()
@@ -91,8 +93,6 @@ def get_ack_endpoint(ack_id: int) -> dict:
     # the regenerated text to keep payloads small; detail does.)
     if row.raw_json:
         try:
-            from cyclone.parsers.models_999 import ParseResult999
-            from cyclone.parsers.serialize_999 import serialize_999
             regenerated = ParseResult999.model_validate(row.raw_json)
             icn = regenerated.envelope.control_number or "000000001"
             body["raw_999_text"] = serialize_999(regenerated, interchange_control_number=icn)